This invention relates to compositions of matter classified in the art of chemistry as fluoropolymers, more specifically as copolymers of vinylidene fluoride (VDF), more specifically as copolymers of vinylidene fluoride and hexafluoropropylene (HFP), still more specifically as copolymers of VDF and HFP having reduced extractable content, longer gel times and improved solution clarity relative to VDF, HFP copolymers known previously which otherwise have similar proportions of HFP in the bulk polymer, to novel compositions of matter and articles of manufacture containing such copolymers, as well as to processes for the preparation and use of the copolymers, of compositions of manufacture containing such copolymers and of the articles of manufacture containing such copolymers.
VDF/HFP copolymers are well known and are used for their thermoplastic engineering properties, chemical resistance and inertness toward degradation. They may be found in applications such as chemically resistant piping, gasketing, plenum cable jacketing, filtration and extraction membranes and in the construction of lithium batteries.
The present invention provides VDF/HFP copolymers containing up to about 24 weight % (12 mole %) HFP having among other improved properties, reduced melting point (at HFP weight percent content of 8% or greater) when determined by differential scanning calorimetry, substantially improved solution clarity, longer gel times and reduced extractables as these terms are defined hereinafter.
The process used to make the instant copolymers requires one ratio of VDF and HFP for the initial fill of the reactor, and a different ratio of VDF and HFP during a subsequent continuous feed of the monomers. Any particular desired average HFP content in the copolymer product has corresponding particular initial fill and subsequent feed ratios. The uniformity of compositions prepared this way provide unique and useful properties in comparison to all VDF/HFP copolymers previously known. These differences are not due to the presence or absence of foreign material impurities.
The present invention also provides lithium batteries fabricated from the VDF/HFP copolymers of the present invention which are uniquely suitable for lithium battery construction.
Rexford in U.S. Pat. No. 3,051,677 described VDF/HFP copolymers of HFP content 30 to 70 wt % (15 to 50 mol %) which showed utility as elastomers. To make the copolymers, a batch process with certain initial ratios of VDF and HFP, and also a continuous process with fixed ratios of VDF and HFP throughout the process were described. The polymers produced had HFP ratios higher than those contemplated by the present invention and they were made differently.
Lo in U.S. Pat. No. 3,178,399 described VDF/HFP copolymers of HFP content of 2 to 26 wt % (1 to 13 mol %) which showed a numerical value for the product of the tensile strength (psig) and percent reversible elongation of at least 1,000,000. A batch process with certain initial ratios of VDF and HFP, or, alternately, a semicontinuous process with fixed ratios of VDF and HFP throughout the process were used to make the copolymers. The processes discussed were such that copolymers made according to these processes have higher melting points and lack the improved solution clarity, longer gel times and low extractables of the present invention copolymers.
Barber in U.S. Pat. No. 5,093,427 describes a synthetic method and copolymer produced thereby which are the exact antithesis of the synthetic method and copolymers of the present invention. The synthesis in Barber is intended to and has been demonstrated to produce copolymers having significantly higher melting points as well as other polymer properties such as solution clarity, solution stability and extractable content markedly different from the polymers of this invention.
Moggi, et al. in Polymer Bulletin 7, 115-122 (1982) analyzed the microstructure and crystal structure of VDF/HFP copolymers by nuclear magnetic resonance and x-ray diffraction experiments. The copolymers of up to 31 wt % (up to 16 mol %) HFP were made in a batch emulsion process which was carried only to low conversion. While a low conversion batch process is theoretically capable of producing copolymers having lower melting points, solution clarity and low extractables, no such properties are described. It is not a practical process for industrial use because of the low conversions required to make the materials. In addition, since no detailed polymerization examples were offered, it is not possible to reproduce the polymers tested with any degree of certainty.
Bonardelli et al. in Polymer, vol. 27, 905-909 (June 1986) studied the glass transition temperatures of VDF/HFP copolymers having HFP content up to 62 wt. % (up to 41 mol %). The glass transition temperatures were correlated to the overall HFP content in the copolymers. In making the copolymers for analysis, a semicontinuous emulsion process was used which employed different VDF/HFP ratios for the initial fill of the reactor and for the subsequent continuous feed of monomers. Although reference was made to the use of reactivity ratios to set the VDF/HFP ratio for the initial fill, no detailed polymerization examples were offered, and no mention of copolymers having solution clarity, gel times (solution stability) and low extractables comparable to that of the copolymers of the present invention was made.
Pianca et al. in Polymer, vol. 28, 224-230 (February 1987) examined the microstructure of VDF/HFP copolymers by nuclear magnetic resonance, and the microstructure determinations were used to explain the trend in glass transition temperatures of the copolymers. The synthesis of the copolymers involved a semicontinuous emulsion process which used different VDF/HFP ratios for the initial fill of the reactor and for the subsequent continuous feed of monomers. No detailed synthesis examples were provided, and there was no discussion of copolymers having lowered melting points, improved solution clarity, longer gel times and low extractables as provided by the copolymers of the present invention.
Abusleme et al. in Eur. Pat. Appl. No. 650,982 A1 showed an emulsion process to make polymers and copolymers of fluorinated olefins optionally with one or more non-fluorinated olefins. The process relied on photochemical initiation of polymerization so that lower temperatures and pressures could be used than those used for thermally initiated processes. While there was general mention of the structural regularity of the resulting polymers, the only evidence of regularity concerned poly(vinylidene fluoride) homopolymer, and no claims were made as to regularity of composition. Examples of VDF/HFP copolymerization were given, but no discussion of the melting points and/or of the solution extraction properties of the copolymers was given, and there was no relation made between physical properties and the structure of the VDF/HFP copolymers.
Morgan in U.S. Pat. No. 5,543,217 disclosed uniform tetrafluoroethylene/hexafluoropropylene copolymers (TFE/HFP copolymers) made by a semicontinuous emulsion process. Uniformity was simply defined as there being a low proportion of adjacent HFP units in the polymer chains; there was no disclosure of the disposition of TFE and HFP units otherwise, and there was no discussion of VDF/HFP copolymers or the properties to be expected therefrom.
U.S. Pat. No. 2,752,331 describes the synthesis of VDF/chlorotrifluoroethylene (CTFE) copolymers having a high uniformity of comonomer distribution in its molecular chains.
Baggett and Smith in High Polymers, Vol. XVIII, Ham, John Wiley (1964), Chapter X, Copolymerization, pp. 587 et seq., particularly at pp. 593 and 610 describe the sythesis of uniform composition distribution copolymers of vinylidene chloride and vinyl chloride and of vinyl chloride and vinyl acetate.
None of these references teaches or suggests a way to obtain VDF/HFP copolymers having lower melting temperatures (when the copolymer has greater than 8 wt. % HFP content), solvent solution clarity and fluidity, longer gel times and low extractables when compared to the previously known VDF/HFP copolymers described by such references all of which have comparable freedom from extraneous impurities.
U.S. Pat. No. 4,076,929 describes the synthesis of VDF homopolymer having a uniformly distributed relatively high defect structure in its molecular chains.
U.S. Pat. No. 5,296,318 teaches that only VDF/HFP copolymers having 8 to 25% by weight HFP and no other VDF homo- or copolymers are suitable for use in fabrication of lithium battery electrodes and separators.
U.S. Pat. No. 5,348,818 mentions that among other polymers, VDF polymer may be used in forming a solid electrolyte for use in secondary battery manufacture. No particular type of polyvinylidene fluoride is identified and no copolymers thereof of any type are suggested.
European Patent Application 95 120 660.6-1215 published Sep. 4, 1996, corresponding to U.S. Pat. No. 5,922,493, teaches use of micro porous (open or closed cell) VDF copolymers of about 7% to about 25% comonomer content in lithium batteries and the use of VDF homopolymer as a cladding material for such batteries. Use of copolymers used by the instant invention in solid electrolytes and the improved properties provided thereby are not taught or suggested. Similarly the use of the type of VDF homopolymer used in the present invention in solid electrolytes is not taught of suggested.
None of these references teach or suggest lithium rocking chair batteries of the type contemplated by the present invention and U.S. Pat. No. 5,296,318 expressly teaches away from use of VDF homopolymer of any type, VDF/HFP copolymers of less than 8% by weight HFP content or other VDF containing copolymers.
U.S. Pat. No. 5,571,634 teaches a lithium ion battery construction employing a VDF-CTFE copolymer where the CTFE content in the copolymer is no less than 8% by weight.
The invention provides in a first composition aspect a copolymer of vinylidene fluoride and hexafluoropropylene containing a maximum of about 24 weight percent hexafluoropropylene, having a uniform distribution of hexafluoropropylene residues between chains, such uniformity being demonstrated by the copolymers having solutions of improved clarity and fluidity, such uniformity additionally being demonstrated by:
i) for the copolymers having up to about 8 weight percent nominal HFP content, having weight percent extractables within plus or minus 1.5% of the percent by weight extractables calculated by an equation selected from the group consisting of:
a) Wt. % Extractable=1.7(HFP mole %)xe2x88x923.2, and
b) Wt. % Extractable=xe2x88x921.2+1.5(HFP mole %)xe2x88x928xc3x9710xe2x88x926(Mn) when HFP content is determined by Method A described below, where Mn is number-average molecular weight, and
ii for the copolymers having 8 weight percent nominal HFP content and greater, having a DSC (differential scanning calorimetry) melting point at least 2.5xc2x0 C. lower than the DSC melting point of copolymers having the same nominal weight percent HFP prepared by processes reported in the prior art when the HFP content is determined by Method A described below and having DSC melting points which fall on or below a value determined by the equation:
DSC melting point in deg. C=166.7xe2x88x922.9(HFP wt %), 
when the HFP content is determined by Method B
described below.
The tangible embodiments of this first composition aspect of the invention are straw colored to colorless semi crystalline solids having melting points, as determined by differential scanning calorimetry (DSC), lower than VDF/HFP copolymers having the same nominal HFP percentage content prepared by processes reported in detail in the prior art.
The tangible embodiments of this first composition aspect of the invention also possess longer gelation times from solution as well as greater clarity and fluidity of solutions containing them than VDF/HFP copolymers having the same nominal HFP content prepared by processes reported in detail in the prior art.
The processes reported in detail in the prior art are those identified in the prior art section hereinabove which are stated to provide sufficient information for accurate reproduction of their experimental methods. Specifically, they are U.S. Pat. No. 3,178,399 and U.S. Pat. No. 5,093,427 all of which are concerned with VDF, HDF copolymers. The DSC of the copolymers produced by the methods of these references containing HFP content of 8 weight percent or greater up to about 24 weight percent fall above the value determined by the DSC melting point equation stated above.
The aforementioned physical characteristics taken together with the method of synthesis positively tend to confirm the structure and the novelty of the compositions sought to be patented.
The tangible embodiments of the first composition aspect of the invention have the inherent applied use characteristics of being suitable for paint and powder coating vehicles and as chemically resistant shaped objects and films both supported and unsupported. Particular mention is made of copolymers of the first composition aspect of the present invention having from about 2 weight % HFP content to 8 weight % HFP, more particularly copolymers having about 3 to 7 weight % HFP, still more particularly those having 3 to 6 wt % HFP which possess the inherent applied use characteristics of being particularly suitable as polymeric separators and polymeric electrode matrices for batteries, particularly lithium batteries.
The prior art, see for example U.S. Pat. No. 5,296,318, has reported lithium batteries made from VDF/HFP copolymers having from 8% to 25% by weight HFP. It is understood that the copolymers of the present invention having HFP content in that range are also suitable for use in such batteries and would represent an improvement therein. Such improved batteries are also contemplated by the invention as equivalents.
Particular mention is also made of copolymers of the first composition aspect of the invention having from about 7 weight percent HFP content to about 15 weight percent HFP content, more particularly copolymers having about 10 weight percent HFP content which possess the inherent applied use characteristic of being suitable as flame resistant insulation for wire and cable.
Still further mention is made of copolymers of the first composition aspect of the invention having greater than about 15 weight percent HFP content, still more particularly of copolymers having about 16% by weight or greater HFP content which have the inherent applied use characteristic as clear, flexible, chemically resistant films.
The invention provides in a second composition of matter aspect, an improved article of manufacture comprising an electrochemical cell having a positive electrode, an absorber separator and a negative electrode wherein at least either one of the electrodes comprises a vinylidene fluoride polymer having an electrolyte material combined therewith and/or said absorber separator comprises a vinylidene fluoride polymer having an electrolyte material combined therewith wherein the improvement comprises the polyvinylidene fluoride polymer consisting essentially of a vinylidene fluoride/hexafluoropropylene copolymer as defined in the first composition aspect of the invention.
Special mention is made of embodiments of the second composition of the invention wherein the VDF/HFP copolymer has a hexafluoropropylene content of from about 2 wt % up to 8 wt % hexafluoropropylene, particularly those having from 3 weight % to 7 weight % hexafluoropropylene, still more particularly, those having about 3 weight % to 6 wt % hexafluoropropylene.
As used herein and in the appended claims, vinylidene fluoride polymers (or VDF polymers) of the present invention means the VDF/HFP copolymers which are the first composition aspect of the invention.
The electrochemical cells of the type of which the second composition of matter aspect of this invention is an improvement are described in PCT Application WO 95/06332, European Patent Application 95 120 660.6-1215, published as number 730,316 A1 on Sep. 4, 1996 and U.S. Pat. No. 5,296,318. The disclosures of the PCT application, the European application and the U.S. patent are hereby incorporated by reference.
In addition to use of solution casting techniques for preparation of films for use in battery constructions as described in the aforementioned references, use of extrusion techniques to prepare such films and the batteries fabricated therefrom are also contemplated.
It has also been noted that batteries fabricated from the above described VDF polymers, particularly the VDF-HFP copolymers of the present invention have better adhesion of the polymers to metallic portions of electrodes and higher use temperatures than batteries fabricated from VDF-HFP copolymers of the prior art. It has also been observed that VDF-HFP copolymers of the present invention provide batteries having improved electrical properties including the capability of higher discharge rates than batteries fabricated from VDF-HFP copolymers of the prior art. It is expected by the present inventors that in general batteries fabricated according to the present invention will possess such higher temperature use and higher discharge rate capabilities.
The invention provides in a third composition aspect, a solution of a composition of the first composition aspect of the invention having improved solution clarity and fluidity.
Copolymers of vinylidene fluoride and hexafluoropropylene of up to about 24 wt % hexafluoropropylene are useful semicrystalline thermoplastics. As the HFP content increases in the materials, the crystallinity decreases, and, correspondingly, the flexibility and solvent sensitivity increase. Other properties change as well, such as the final melting point, which decreases with increasing HFP content. In high-purity applications such as membrane filtration or extraction, lithium battery construction, high-transparency film from solution casting, and fluid storage and transport requiring low contaminant levels, it is desirable to have materials with low levels of extractables, little gel formation in the presence of solvent, and good clarity. The VDF/HFP copolymers provided here show lower extractables, improved solution properties, improved clarity and fluidity, and lower melting points in comparison to the nonuniform VDF/HFP copolymers of otherwise similar HFP content whose manufacture is enabled in the prior art.
By uniform it is meant that there is decreased chain to chain variation in the distribution of HFP residues in polymer chains in comparison to the nonuniform copolymers of the prior art where the HFP content from chain to chain varies broadly, thereby imparting measurably different inherent physical and chemical properties.